Naranjo ADR Probability Scale
Naranjo CA, Busto U, Sellers EM, Sandor P, Ruiz I, Roberts EA, et al. A method for estimating the probability
of adverse drug reactions. Clin Pharmacol Ther 1981; 30 (2): 239-45.
Note to authors: Please see page 5 for an example of how to report the score obtained from the scale
in your manuscript. Thank you.
Question
Yes
No
Don't know
Are there previous conclusive reports on this reaction?
Did the adverse event appear after the suspected drug
was administered?
+1
+2
0
-1
0
0
Did the adverse reaction improve when the drug was
discontinued, or a specific antagonist was administered?
4. Did the adverse reaction reappear when the drug was
readministered?
5. Are there alternative causes (other than the drug) that could
on their own have caused the reaction?
6. Did the reaction reappear when a placebo was given?
7. Was the drug detected in the blood (or other fluids) in
concentrations known to be toxic?
8. Was the reaction more severe when the dose was increased,
or less severe when the dose was decreased?
9. Did the patient have a similar reaction to the same or similar
drug in any previous exposure?
10. Was the adverse event confirmed by any objective evidence?
+1
0
0
+2
-1
0
-1
+2
0
-1
+1
+1
0
0
0
+1
0
0
+1
0
0
+1
0
0
1.
2.
3.
1
Score
Instructions for using the ADR probability scale
Used with permission from the School of Pharmacy, University of North Carolina at Chapel Hill; we acknowledge
the unique additions by Dr. John M. Kessler, Dr. Bashar and the University of North Carolina.
General Instructions:
Collect all the relevant information about the alleged ADR before using the questionnaire.
Please read each question carefully. If you do not understand a particular question, consult
the specific instructions given below. Use the "do not know" answers only when the quality
of the data does not allow an affirmative (yes) or negative (no) answer. Similarly, you should
answer "do not know" if the information is unavailable or the question is inapplicable. Some
minor modifications are required in certain circumstances. For analyzing the adverse drug
interaction, the suspected interacting drugs, instead a of particular drug, must be assessed.
When the patient is receiving several drugs, the ADR probability scale must be applied to
every one of the suspected etiological agents. The presumed etiology will be the drug with
the highest score. In reactions which appear during drug withdrawal, dechallenge (Question
3) refers to reinstituting treatment with the suspected drug whereas rechallenge (Question 4)
consists of discontinuing the alleged drug. Similarly, Question 2 must be reworded: Did the
adverse effect appear after the suspected drug was discontinued? (See ADR Probability
Scale: Drug Withdrawal.)
Specific Instructions:
Question 1:
It assesses the previous general experience with drug use. You should answer "yes" only if
there are two or more published reports which have previously described in detail the adverse
clinical event (ACE). Similarly, you can accept as a reliable source of information the listing
of ACE in a classic textbook on adverse reactions such as Meyler's Side Effects of Drugs,
Vol. 9, MNG Dukes, Amsterdam, Excerpta Medica, 1979. You should answer "no" when the
ACE has not been previously described. The answer "do not know" is only applicable when
the information is either unavailable or inconclusive.
Question 2:
It assesses the time sequence of the reaction. The answer must be "yes" only if there is
definite evidence that the ACE occurred after the suspected drug was administered. "No" is
applicable when the ACE developed prior to drug administration. You must answer "do not
know" if the information available does not allow certain decision.
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Question 3:
It assesses the effect of dechallenge. The answer must be "yes" only if the ACE diminished or
disappeared at any time after discontinuation of the suspected drug. Similarly, you can
answer "yes" if the reaction disappeared after specific pharmacologic antagonist was
administered (e.g., an anticholinergic drug, such as atropine, antagonized the effect of a
cholinergic drug, such as physostigmine). You must answer "no" if the reaction did not
disappear or did not diminish after drug discontinuation. Similarly, you must answer "no" if
the ACE improved after a non-specific pharmacologic antagonist was administered (e.g.,
potassium salts in a diuretic-induced hypokalemia). You must answer "do not know" in the
following circumstances: the suspected drug was not discontinued, the information is
unavailable, or the results of dechallenge are inclusive. If the ACE is transitory (e.g.,
convulsions), and the clinical circumstances do not allow an unequivocal assessment of the
effect of dechallenge, the answer must be "do not know."
Question 4:
It assesses the effect of rechallenge. You must answer "yes" only if the following conditions
were met: a) the suspected drug was discontinued, b) a new baseline of the ACE was
determined, c) there was an unequivocal reappearance when the drug was readministered in
similar doses and by the identical route. Similarly, a positive answer can be given in those
cases in which the other factors suggest a strong causal association and the ACE is a wellknown effect of a drug (e.g., furosemide-induced hypokalemia) and for clinical and/or ethical
reasons the rechallenge is unwise. You must answer "no" when one or all of the conditions of
rechallenge listed in a, b, and c were not met. You must answer "do not know" if the
information is either unavailable or equivocal.
Question 5:
It assesses alternative etiologic candidates of the ACE. Since ADR usually are nonspecific
manifestations of disease, you must carefully consider the eventual contribution of etiologies
other than the suspected drug. You must answer "no" only if after a systematic and
exhaustive investigation no alternative causes were detected, thus implying the suspected
drug more strongly. You must bear in mind that a contributing factor to the ACE does not
qualify as an alternative etiologic candidate (e.g., if a patient with prior renal failure develops
an impairment of renal function secondary to therapy with gentamicin, the prior renal failure
is a contributing factor, not an alternative etiology of the ADR). You must answer "yes" if
there is an alternative etiologic candidate. You must answer "do not know" if the
investigation of other causes was incomplete, inconclusive, or not performed.
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Question 6:
It is important when the ACE is subjective. The answer must be "yes" only if the following
conditions were met: a) The suspected drug was discontinued, b) a new baseline of the
clinical manifestation was established, and c) there was an unequivocal reappearance of the
ACE after single- or double-blind placebo administration. The answer must be "no" if readministration of placebo, as described above, did not elicit the same reaction. You must
answer "do not know" when a placebo test was not performed, the information is unavailable,
or placebo effect is inclusive.
Question 7:
It is very important in cases of dose-related ADRs, and it assesses the information provided
by drug concentrations in blood, other body fluids, or tissues. The answer must be "yes" only
if the concentration of the alleged drug is in accepted toxic range. The answer must be "no" if
the drug concentration is below the toxic range, and the answer must be "do not know" if this
information is either unavailable or the question is inapplicable (i.e., in ADRs which are not
dose related).
Question 8:
It assesses clinically the dose-response relationship. You must answer "yes" only if the ACE
was more severe when the dose of the alleged drug was increased. Similarly, you must
answer "yes" if the ACE was less severe after reduction in the dose of the alleged drug. The
answer must be "no" if there was no clinically detectable variation in the ACE when the dose
of the suspected drug was changed. You must answer "do not know" when either the dose did
not change or the information is unavailable.
Question 9:
It assesses the personal experience of the patient in previous administrations of the same
chemically related drugs. You must answer "yes" only if the patient reports a similar ACE to
the drug(s). Similarly, you must answer "yes" if there is documentation in the patient's record
of such prior reactions (e.g., documentation of prior manifestation of allergy to penicillin,
such as skin rash or anaphylaxis.) You must answer "no" when either the patient has no
previous exposure to the drug or if in a previous exposure to the same or similar drugs the
patient did not develop the ACE. The answer must be "do not know" when either the
information is unavailable or it is inclusive.
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Question 10:
It assesses the quality of the information in which we are supporting our judgments. This is
crucial because even though the patient's report is a very important source of information,
some patients can be unreliable, for instance, because of memory impairment. You must
answer "yes" only in the following circumstances: a) a laboratory test documented the ADR
(e.g., in the case of suspected drug-induced hepatitis, a compatible liver biopsy), b) the ACE
was directly observed by qualified personnel (e.g., the attending physicians or nurses
observed manifestations such as the skin rash, jaundice, etc.). The answer must be "no" when
either the laboratory tests or the direct clinical observation did not document the reaction. The
answer must be "do not know" when the ACE is subjective, the information is unavailable or
it is inclusive.
Authors: Please include the results of your Naranjo ADR Probability Scale
evaluation in the main text of your manuscript and the abstract (if applicable).
While it is not necessary to cite the numerical score, a statement of the
assessment of the likelihood (e.g., > 9 = highly probable, 5 - 8 = probable, 1 - 4 =
possible and  0 = doubtful) that the adverse reaction or drug interaction was
drug-related should be stated.
An example of how such a sentence might read could be as simple as “Use of the
Naranjo ADR Probability Scalereference number of Naranjo article indicated a ‘highly
probable/probable/possible/doubtful’ ((insert appropriate word depending on the
score recorded from the Scale)) relationship between the adverse effect ((specify
which adverse effect)) and ((name of drug)) therapy in this patient.”
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